As known in the art, a signal amplitude of ECG signals is typically in the order of 1 mV, but may have a DC offset that varies from as much as −300 mV to +300 mV. This DC offset may drift with time and/or patient movement, and is often referred to as a “baseline wander”. Additionally, events such as defibrillation may have a dramatic effect on the baseline. In particular, a DC offset following a defibrillation event is usually drifting due to current that may flow through the ECG electrodes during the defibrillation event.
A typical ECG signal display setting for gain has a range of +/−2 mV in order to visually see a 1 mV ECG signal clearly. In response to potentially large and drifting DC offsets, high pass filters have been utilized to remove any DC offset in order to keep the ECG signal within view windows of a display and a printer. More particularly, a key diagnostic measurement of a ECG signal is the ST segment elevation or depression. This is performed by comparing a baseline of the ECG signal prior to a QRS with the baseline after the QRS. Ideally, the high pass filter should remove the baseline wander in such a way that the relative level of the baseline before and after the QRS is not affected.
ECG standards have been established that describe an impulse response requirement for diagnostic quality ECG measurements (e.g., EN 60601-2-27 and AAMI EC13). For example, an impulse applied in a standard test is 3 mV in amplitude with a duration of 100 mS, and the requirement is that a baseline should be displaced by less than 100 uV and a slope of the baseline should be less than 300 uV/sec following the impulse. Therefore, a high pass filter in an ECG system has conflicting goals.
Specifically, if the high pass filter is very responsive to the baseline wander in order to reliably maintain the baseline of the ECG signal in the center of the display, then it will also likely be responsive to the QRS such that the baseline following the QRS is displaced following the QRS by more than 100 uV. This is why an ECG monitor usually provides the clinician with several bandwidth settings for the high pass filter. The settings are often referred to as “Monitor” bandwidth for keeping the ECG signal visible on the display screen, and as “Diagnostic” bandwidth for making diagnostic ECG measurements (e.g., ST segment elevation and depression). Additionally, there is also the desire to display the ECG signal in real time with minimal time delay. This is important for clinical applications where timing is important such as synchronized cardioversion.
Historically, several types of high pass filters have been utilized in ECG monitors.
One such type of high pass filter for ECG monitors is an infinite impulse response (“IIR”) high pass filter that is computationally simple to implement. For example, a second order Butterworth high pass filter is easily implemented with five (5) multiply and accumulate calculations per sample with minimal time delay. However, a disadvantage of a BR high pass filter is that a group delay is frequency dependent. This results in distortion of the ECG signal. Stated in another way, a BR high pass filter responds to a positive ECG QRS signal by depressing the baseline following the ECG signal. Furthermore, in order to minimize the distortion to a level acceptable for diagnostic purposes, the corner frequency of the IIR high pass filter needs to be reduced to a frequency of 0.05 Hz or less. Additionally, a first order IIR high pass filter applied to a ramp will result in a DC offset and a second order IIR high pass filter applied to a ramp will result in a zero (0) DC offset. Thus, in order to remove a DC offset that is drifting following a defibrillation event, the IIR high pass filter would need to be at minimum a second order filter.
Another type of high pass filter for ECG monitors is a finite impulse response (“FIR”) high pass filter, which by definition has linear phase and constant group delay. Of note, a FIR high pass filter minimizes the distortion of the ECG signal due to the constant group delay and a 0.5 Hz or even a 0.67 Hz FIR high pass filter maybe implemented that meet the requirements for diagnostic quality ECG measurements in accordance ECG standards. Also, a FIR high pass filter responds well to a drifting DC offset following defibrillation, because it is usually designed to be symmetrical and an application of a FIR high pass filter to a ramp will produce a zero (0) DC offset. However, there are a couple of disadvantages of the FIR high pass filter. The first disadvantage is the time delay. Specifically, in order to have constant time delay for all frequencies, both the frequencies above and below the high pass corner frequency will see the same time delay, and a typical time delay is on the order of about one (1) second. The second disadvantage is the computational effort required. Specifically, a FIR high pass filter with one (1) second of time delay will have two (2) seconds of time history. A sample rate of 1000 Hz would require 2000 multiply accumulate calculations for each sample calculated at the 1000 Hz sample rate. Thus, for a full twelve (12) lead measurement, the number of multiply accumulate operations is 24M just for the FIR high pass filter.
Moreover, ECG monitoring is often performed on patients that are being moved. The out of hospital emergency medical services (“EMS”) typically see significant baseline wander of the ECG due to the movement of the patient. An EMS High pass filter is often provided for ECG systems designed for the EMS environment. This high pass filter will typically have a corner frequency in the range of 1 Hz to 2 Hz. A simple IIR filter with this high a corner frequency very substantially distorts the ECG waveform. A FIR filter with this corner frequency will minimize distortion of the ECG but would require a significant increase in computational effort.